Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). Multi-fuel engine systems can use both port and direct injection with different fuel types provided to the different injectors. For example, direct injection of ethanol fuel may be used with port injection of gasoline fuel. Therein, the direct injection of the alcohol fuel may take advantage of the increased charge cooling effects of the alcohol fuel's higher heat of vaporization and increased octane. This helps to address knock limitations, especially under boosted conditions. Further, the port injection of the gasoline fuel may take advantage of the higher power output of the gasoline fuel.
As such, the composition of a fuel injected into an engine affects the volumetric fuel economy and efficiency of the engine. As recognized by Surnilla et al. in U.S. Pat. No. 8,387,591, the non-linear relationship between the volumetric composition of a direct injected fuel and the octane number of the fuel can make calculation of engine control adjustments complex. The inventors herein have recognized that in multi-fuel systems, such as a dual fuel system, a further degree of complexity is involved. This is because the volumetric efficiency of the engine depends on the net effect of the types of fuel injected into the engine, the amount of each fuel that is injected, as well as the injection system used to inject the fuel. In other words, the impact on volumetric efficiency when a gasoline fuel is port injected and an ethanol fuel is direct injected may be different from when the gasoline fuel is direct injected and the ethanol fuel is port injected. As such, if the volumetric efficiency of the engine is not adjusted to correct for these fuel effects, the amount of air in the cylinder may be estimated erroneously. This would, in turn, lead to errors in engine torque estimation, fuel system monitor errors, etc. In sum, engine performance would be degraded.
At least some of the above issues may be at least partly addressed by a method for accurately estimating engine volumetric efficiency when operating with a multi-fuel system. The method includes: adjusting an estimate of engine volumetric efficiency in response to fuel port injected and fuel direct injected into a cylinder during a cylinder cycle; and adjusting an actuator in response to the estimate of engine volumetric efficiency. In this way, cylinder aircharge estimation errors can be reduced.
As an example, an engine may be fueled with a first fuel (e.g., a primary fuel, such as gasoline) via port injection. The engine may also be fueled with a second, different fuel (e.g., a secondary fuel, such as ethanol) via direct injection. The engine volumetric efficiency of the dual fuel engine may be determined based on the injection amount of each of two fuels as well as their injection system. In particular, the volumetric efficiency estimate may be reduced as the port injection of the gasoline fuel increases while the volumetric efficiency estimate may be increased as the direct injection of the ethanol fuel increases. The overall volumetric efficiency of the engine is calculated as the net effect of the volumetric efficiency decrease due to port injection of the gasoline fuel fraction and the volumetric efficiency increase due to direct injection of the ethanol fuel fraction. As such, when fuel evaporates in the manifold or the cylinder (in the intake stroke), it contributes a pressure which is the partial pressure of the fuel. Therefore, the effect of the port injected gasoline fraction may be determined based on a partial pressure of the gasoline fuel in the intake port. The effect of the direct injected ethanol fuel may be likewise determined based on the partial pressure of the ethanol fuel in the cylinder. In particular, since the cooling effect of the direct injected ethanol fuel is achieved only during the intake stroke while the intake valve is open, the adjusting may be based on the partial pressure in the cylinder during an IVO event. In one example, a controller may determine an initial or base volumetric efficiency estimate based on engine operating conditions, such as engine speed-load conditions, and may further update the base estimate based on the fuel fractions of the different fuels and their injection types. Based on the updated volumetric efficiency estimate, a cylinder aircharge estimate may be corrected. One or more engine actuator adjustments may be accordingly made. For example, one or more of a throttle opening, cam timing, valve timing, spark timing, injected fuel quantity and EGR flow rate may be adjusted.
While the above example is depicted with reference to gasoline and ethanol liquid fuels, it will be appreciated that the fuel type and injection type based volumetric efficiency estimation model may be similarly applied to various other fuel and injection combinations. For example, the same model may be used to accurately adjust the volumetric efficiency in a multi-fuel engine system operating with liquid fuel and gaseous fuel, such as an engine system having port injected CNG and direct injected gasoline.
In this way, the net effect of cooling and partial pressure of each fuel and each fuel injection type of a multi-fuel engine system may be better learned and used to correct a volumetric efficiency estimate. By improving the accuracy of the volumetric efficiency estimate, cylinder aircharge estimation errors can be reduced. As such, this improves engine actuator control and reduces torque disturbances. In addition, errors in the estimation of one or more other engine operating parameters that are based on the air flow estimate are also reduced. By reducing air-fuel errors, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.